Method of nitrating cellulose



Nov. 27, 1934. s, A. POLLQCK I 1,982,122

METHOD OF NITRATING CELLULOSE Filed April 12, 1932 5 m v MM, M 0

Patented Nov. 27, 1934 UNITED STATES PATENT OFFFWE Application April 12, 1932, Serial No. 604,854 In Great Britain March 18, 1930 9 Claims.

The present invention consists of improvements in or relating to methods of nitrating cellulose and an object of the invention is to produce cheaply and conveniently a nitrated product in the form of separate relatively thin sheets having very uniform physical and chemical characteristics.

The invention is particularly applicable to the production of thin sheets of nitrated cellulose for use as separators in electric batteries and for making solutions of nitro-cellulose, for example, for producing acid-resisting varnishes.

According to the present invention a method of preparing separate relatively thin sheets of nitrated cellulose comprises placing in a nitrating bath relatively thin sheets of cellulose fibre paper interleaved with acid-proof spacing sheets provided with projections and recesses giving the nitrating fluid access to both sides of the sheets of cellulose.

Preferably the cellulose sheets are in the form. of pure cotton fibre paper and are not more than 100th of an inch thick while the spacers are conveniently of aluminium, for example, corrugated sheet aluminium. Preferably the sheets lie in vertical or substantially vertical planes when immersed in the bath.

In one form of the invention the interleaved sheets and spacers are assembled in a box or cage capable of being lowered into and raised out of a nitrating bath and provided with apertures to allow the nitrating fluid to flow freely into the box among the sheets.

By means of the invention a nitrated cellulose paper is produced of which the composition is very uniform. It is found that ordinary methods of introducing paper into nitrating baths are liable to result in great variation of nitration and porosity which greatly affects the suitability of the product for use as battery separators and its complete solubility in ordinary nitro-cellulose solvents.

It is found that important factors in the process are the simultaneous access of nitrating acids to both sides of the paper, rapid and complete penetration of acid into all the fibres of the paper, rapid absorption by the concentrated acid of the water produced by the chemical reaction with the cellulose, control of the heat produced by the chemical reaction, and rapid removal of the acid from the nitrated paper at the end of the nitration. Spacers of aluminium besides being substantially acid-resisting assist substantially in the distribution of the heat produced by the reaction.

An apparatus suitable for carrying out my invention is illustrated in the accompanying drawmg.

As shown in the drawing the box 10 which is of aluminium is of rectangular form with a perforated bottom 11, perforated end walls 12, and handles 15. Single sheets 13 of paper are interleaved with thin corrugated aluminium sheets 14 of approximately the same dimensions and placed in the box parallel with its side walls. The box is designed to hold a convenient number of sheets, for example 120, standing on edge on the bottom of the box. 1 I

The box containing the paper and aluminium sheets is placed for about an hour in a drying oven maintained at a temperature slightly above 100 degrees centigrade until the paper is thoroughly dried. The box with its contents is then removed from the oven and when cooled to a temperature of about 15 degrees centigrade is submerged for about 30 minutes in a bath containing a mixture of sulphuric and nitric acids and water of the following approximate co1npositionz- Sulphuric acid 44.3 per cent., nitric acid 37.4 per cent., water 18.2 per cent. If sulphuric acid of 1.84 specific gravity and nitric acid of 1.42 specific gravity are used the above composition is obtained by taking two parts of sulphuric acid to three parts of nitric acid. The chemical reaction which takes place between the acids and the cellulose is principally controlled by the concentration of the acid in the bath and the temperature of the bath. I find that when the bath is maintained at a temperature of about 15 degrees centigrade and a sufficient volume of acid is used to completely submerge the aluminium box and its contents the paper acquires in the time mentioned a definite increase in weight of about 55 per cent., when weighed after complete washing in water and drying. It also acquires the valuable qualities of great strength in the wet condition and a high degree of toughness and flexibility which it retains indefinitely in battery acid of the usual strength, extremely low in electrical resistance when immersed in the battery electrolyte, and a favourable degree of porosity which completely prevents the finest sediment escaping from sheaths made from this material or causing a bridge or short-circuit between oppositely charged plates.

The satisfactory working and maintenance of the acid bath in order to effect economy of acids and obtain a uniform product which is ol' great importance requires detailed explanation. The compound of cellulose esters which is formed with the acids reaches an equilibrium with the concentration of acid in the bath so that the quantities of cellulose and acids in the bath when equilibrium is reached effect the result obtained,

For example, 1000grams of cellulose combine with about 778 grams of nitric acid under these conditions of concentration and temperature to form about 1565 grams of the particular cellulose ester and about 222 grams of water. The consumption of acid and the formation of water as the result of the reaction necessitate a readjustment of the bath before it can be satisfactorily used again to produce a similar result. On the assumption that 1000 grams of pure cellulose paper is treated at one time in the bath the required adjustment may be calculated as follows:-As a first step 778 grams of nitric acid should be restored to the bath to replace the amount used by the previous batch of paper. Us ing nitric acid of 1.5 specific gravity containing 94 per cent pure acid the amount required is 778x divided by 94:82. grams. The addition of this acid will unavoidably add 6 per cent. of 827 grams of water, is. about 49 grams of water containing the acid. Therefore the bath becomes diluted by the addition of 222-grams of water'from the reaction plus 49 grams introduced by the nitric acid addition. in order to restore the water content to its original value of 18.2 per cent. a sufiicient quantity of mixture of concentrated sulphuric and nitric acids in the original ratio of 44.3 to 37.4 must be added to balance the excess of 271 grams water. This ratio of acids will be obtained by taking sulphuric acid of 1.84 specific gravity (95.6 per cent strength) and nitric acid of 1.500 specific gravity (94 per cent strength) in the following quantitiesz-- 443 X mos-9515:4634

grams sulphuric acid of 1.84 S. G. and

grams nitric acid of 1.5 S. G. The corresponding amount of water to these quantities according to the original ratio is 182 grams but these quantitles only contain 463.4 4.4-:-100 plus grams water. The balance of watertherefore required by these quantities of acid to make up the origin ratio is 182 minus 44.26=137.74 grams, i. e. to balance such an increment of 137.74 grams of water brought into the bath by reaction the above amount of acids would be required, and to balance the actual increment of 271 grams water the foregoing quantities would need to be increased in the ratio of 271 to 137.74.

I have found in practice that some additional water .is unavoidably introduced by incomplete drying of the paper and by absorption of water from the air and also that some additional nitric acid leaves the bath in the form of vapour. To compensate for these variations I have found that the foregoing calculated amounts should be increased by 20 per cent and after a full days operation the bath should be analyzed in known manner to determine the amounts of the acids re" maining in the bath so that an accurate adjustment on the principles explained may be made periodically. Under these conditions the bath may be maintained in a practically uniform statemils and corrugate the paper by passing .it bewider separation'between the platesand consethe bath mechanically by the paper so that the pose of extracting the acid from the paper and eventually placed in running water until completely freed from acid. The first water bath extracts the greater proportion of the acid and when the concentration of acid reaches about 40 per cent. the solution may be run to a storage tank for subsequent recovery of the acids and the series of water baths moved forward to replace the first bath emptied. By adopting the principle of moving the water baths in an opposite direction to the movement of the paper being freed *from acid both water and acids are substantially economized. As soon as the paper is completely freed from acid it maybe conveniently stored in water indefinitely or it may be immediately used without further treatment as battery separators. If, however, it is intended to make use of the treated paper in a dry state it should be further purified by prolonged boiling in water to free it from any organic sulphates which may have been formed by the concentrated acid treatment.

It has been found that on exposure of the aluminium spacers to the atmosphere after withdrawal from the nitrating bath and in the'subsequent water washing process a small amount in the nitrating fluid.

The separators described are suitable for use with plates made from sheet lead weighing one pound per square foot or less but for thicker plates carrying a larger amount of active material it may be desirable to provide .a larger space between plates for electrolyte and in that :case I prefer to use a slightly thicker paper of :say 1 0 tween corrugated metal rollers heated to about degrees centigrade before the paper is treated with the acids described. The corrugations so madein the paper before aciding become stiff and permanent after acid'v treatment and provide 1,5

quently more space for the electrolyte.

Another variation which'I may use is to assemble two or more sheets of paper between each pair of corrugated aluminium sheets before aciding in it. such a manner that the paper sheets are pressed closely together when they enter the acid l bath'm order to cementv the paper sheets in contact with each other permanently together. The eifect' of this process is to provide thicker and stronger separators which can be used to support the :lead plates very effectively.

1. A method of preparing separate relatively thin sheets of nitrated cellulose which comprises placing in a nitrating bath relatively 'thin sheets of cellulose fibre paper' interleaved with acid proof spacing sheets provided with projections and recesses giving the .nitrating fluid access to both sides of the sheets of cellulose. i

2. A method of preparing separate relatively thin sheets of nitrated cellulose which comprises placing in a nitrating bath relatively thin sheets of cellulose in the form of pure cotton fibre paper interleaved with acid-proof spacing sheets provided with projections and recesses giving the nitrating fluid access to both sides of the sheets of paper.

3. A method of preparing separate relatively thin sheets of nitrated cellulose which comprises placing in a nitrating bath relatively thin sheets of cellulose fibre paper which are less than one hundredth of an inch thick interleaved with acid-proof spacing sheets provided with projections and recesses giving the nitrating fluid access to both sides of the sheets of cellulose.

4. A method of preparing separate relatively thin sheets of nitrated cellulose which comprises placing in a nitrating bath relatively thin sheets of cellulose fibre paper interleaved with aluminium spacing sheets provided with projections and recesses giving the nitrating fluid access to both sides of the sheets of cellulose.

5. A method of preparing separate relatively thin sheets of nitrated cellulose which comprises placing in a nitrating bath relatively thin sheets of cellulose fibre paper which when immersed in the bath lie in substantially vertical planes and are interleaved with acid-proof spacing sheets provided with projections and recesses giving the nitrating fluid access to both sides of the sheets of cellulose.

6. A method of preparing separate relatively thin sheets of nitrated cellulose which comprises placing relatively thin sheets of cellulose fibre paper interleaved with acid-proof spacing sheets provided with projections and recesses to give the nitrating fluid access to both sides of the cellulose, in a holder capable of being lowered into and raised out of a nitrating bath and provided with spaces to allow the nitrating fluid to flow freely into the holder among the sheets, and immersing said holder in a nitrating bath.

'7. A method of preparing separate relatively thin sheets of nitrated cellulose which comprises placing relatively thin sheets of cellulose fibre paper interleaved with acid-proof spacing sheets provided with projections and recesses to give the nitrating fluid access to both sides of the cellulose, in a holder capable of being lowered into and raised out of a nitrating bath and provided with spaces to allow the nitrating fluid to flow freely into the box among the sheets, placing the holder with the sheets in it in a drying chamber and subsequently immersing it in a nitrating bath.

8. A method of preparing separate relatively thin sheets of nitrated cellulose which comprises placing relatively thin sheets of cellulose fibre paper interleaved with acid-proof spacing sheets provided with projections and recesses to give the nitrating fluid access to both sides of the sheets of cellulose, in a holder capable of being lowered into and raised out of a nitrating bath and provided with spaces to allow the nitrating fluid to flow freely into the box among the sheets, immersing said holder in a nitrating bath, and, after nitration transferring the holder with the sheets in it to a washing bath.

9. A method of preparing separate relatively thin sheets of nitrated cellulose which comprises placing relatively thin sheets of cellulose fibre paper interleaved with acid-proof spacing sheets provided with projections and recesses to give the nitrating fluid access to both sides of the sheets of cellulose, in a holder capable of being lowered into and raised out of a nitrating bath and provided with spaces to allow the nitrating fluid to flow freely into the box among the sheets, immersing said holder in a nitrating bath, and, after nitration, transferring the holder with the sheets in it to each of a series of washing baths of successively decreasing acid concentration whereof the first is removed on reaching a certain concentration and replaced by a fresh bath at the opposite end of the series.

SAMUEL ALEXANDER POLLOCK. 

